Prepared statements in ASP

I am trying to create a prepared statement in ASP, but am having
problems with creating the parameter object. I do the following

Set fnParam = peopleUpdate.CreateParameter("@firstname", adVarChar,
adParamInput, 50, peopleSourceRS("firstname"))

But this gives the following error on the browser:

Error Type:
ADODB.Command (0x800A0BB9)
Arguments are of the wrong type, are out of acceptable range, or are in
conflict with one another.

The server doesn't seem to like adVarChar and adParamInput. I did try
to create the parameter without arguments and then assign the
properties, but it gives the same error for .Type and .Direction. What
am I doing incorrectly here?

thanks